1. Field of the Invention
The invention concerns a method for the production of polyester by transesterification of at least a dicarboxylic acid dialkyl ester or esterification of at least a dicarboxylic acid with at least a diol, and subsequent pre-condensation and polycondensation in the presence of the usual catalysts as well as an additional co-catalyst, and the use of this polyester to produce films, bottles, molded articles or fibers.
2. Description of the Related Art
The production of polyesters in general takes place by means of the conversion of a diol with a dicarboxylic acid or a dicarboxylic acid ester, e.g., dimethyl ester. Initially the diol diester of the dicarboxylic acid is formed, which is then polycondensed by a single- or multi-staged process at increasing temperatures under diminishing pressure, whereby diol and water are liberated. Compounds of Ti, Mn, Mg, Ca, Li, Co and/or Zn are used as catalysts for transesterification, compounds of Sb, Ti, Ge and/or Sn are used for the esterification, and compounds of Sb, Ti, Pb, Ge, Zn and/or Sn or a zeolite are used for the polycondensation, whereby the quantity of metal in the catalyst used for the polycondensation alone can amount to up to 500 ppm relative to polyester. Antimony compounds, as the most frequently used esterification and polycondensation catalysts in the production of polyester, are required in quantities of approximately 150-250 ppm antimony, but concentrations of more than 200 ppm antimony are undesirable, in particular for using polyester for foodstuffs packagings.
From the literature (Derwent-abstract No. 81-33905 D of SU 759541 B1) it is known that the transesterification of dimethyl terephthalate with ethylene glycol can be carried out in the presence of a mixture of titanium tetrabutylate and activated charcoal at a ratio by weight of approximately 0.017:1, whereby very high quantities of approximately 2 weight % activated charcoal (relative to dialkyl ester) are employed. The activated charcoal serves as an agent to influence the color.
In another method (Derwent abstract No. 76-88266X of SU 495333 A) to produce PVC plasticizers, mixtures of di- and tricarboxylic acid methyl esters are transesterified with neopentyl glycol in the presence of zinc acetate and activated charcoal. According to the patent, approximately 0.6 weight % zinc acetate and approximately 1.2 weight % activated charcoal (relative to methyl ester) are required. A further disclosure (Derwent abstract No. 88-297391 of JP 63 218750 A) specifies the production of low molecular weight plasticizers through co-polymerization of adipic acid and hydroxy stearic acid with butylene glycol and ethyl hexanol in the presence of approximately 0.045 weight % dibutyl tin oxide and approximately 0.9 weight % activated charcoal (relative to the sum of the acids). To date nothing is known, however, as far as a specific catalytic activity of activated charcoal and/or its use as a co-catalyst when polycondensing linear polyester is concerned.
We recognized that prior art methods of producing polyesters resulted in a product having an amount of catalytic metal compounds (and other harmful substances) that would be desirable to reduce to make a safer product for use, for example, in the packaging of foodstuffs. The present invention accomplishes this goal.
The present invention comprises a method of producing polyester for use, in particular, in bottles, films, and miscellaneous foodstuffs packaging as well as filaments and fibers, which polyester has a reduced content of catalytic metal compounds and possibly other substances that may be harmful to health as compared to polyesters produced by prior art methods.
The method of the present invention is characterized by the fact that polycondensation and, optionally, esterification take place in the presence of an additional carbon containing co-catalyst. The carbon containing co-catalyst is preferably activated charcoal with a specific surface area of more than 500 m2/g and an average grain size of less than 2 xcexcm.
The foregoing merely summarizes certain aspects of the present invention and is not intended, nor should it be construed, as limiting the invention in any manner. The invention is described in more detail below.
Surprisingly, it was found that activated charcoal, in conjunction with the usual catalytic metal compounds, acts as a co-catalyst for the polycondensation and/or the esterification reaction. The catalytic effect can be attributed to the properties of the activated charcoal. The basis for the secondary structure of the activated charcoal lies in the given random arrangement of graphite crystalites and amorphous carbon. Gaps and pores are located between these individual particles (micropores less than 2 nm, mesopores 2-50 nm, macropores greater than 50 nm), which form a very great cavity system and result in the large, specific surface area typical of activated charcoal. As a result of the special crystal structure of the activated charcoal, the carbon atoms positioned at the edge of the layer structure are chemically unsaturated and form so called active centers that are the basis for the reactivity of the activated charcoal. Activated charcoal having a specific surface area of more than 500 m2/g (preferably more than 900 m2/g) is suited to function as a co-catalyst.
An elemental analysis reveals that, in addition to hydrogen, oxygen and nitrogen, other inorganic components originating from the raw material of the activated charcoal, such as Ca, K, Na, Si, Fe, Mg, Mn, Zn and Cl, can be found in trace amounts in the activated charcoal. (Hartmut v. Kienle, Erich Baeder; Activated Charcoal and its Industrial Applications, Ferdinand Enke Verlag/Stuttgart, (1980)). The preferred activated charcoal has a quality such as is used commercially for treating drinking water, for the foodstuffs industry, or for medicinal purposes. Such activated charcoal provides the advantage of being free of polycyclic aromatic compounds that are harmful to health, such as benzopyrene, so that the polyester that is produced can be used without problems for foodstuffs packaging.
Since the catalytic activity of the activated charcoal and the coloration of polyester mixed with activated charcoal is to a great degree dependent on the size of the grain, the activated charcoal according to the invention is selected so as to have a grain size (arithmetic mean) d50 of less than 2 xcexcm, preferably less than 0.5 xcexcm. Preferably, the selection of the grain size takes place by milling the powdered activated charcoal in a liquid medium (preferably the diol that is the basis of the polyester, e.g., for polyethylene terephthalate or -naphthalate in ethylene glycol).
The carbon containing co-catalyst is used in addition to the usual polycondensation catalysts, such as compounds of Sb, Ti, Pb, Ge, Zn and/or Sn or a Zeolite, and, optionally, in addition to the usual esterification catalysts, such as compounds of Sb, Ti, Ge and/or Sn, in quantities of 0.1 to 1000 ppm (preferably 0.1 to 500 ppm) co-catalyst relative to polyester at a ratio by weight of catalyst to co-catalyst of 1 to from 0.01 to 5 (preferably 1 to from 0.01 to 3).
As the concentration of co-catalyst increases, the concentration of the usual catalysts used for the polycondensation or esterification can be reduced, whereby preferably 1 to 3 parts by weight of activated charcoal replaces approximately 1 part by weight of the metal of the usual catalyst. The quantity of the usual catalyst that is used together with the co-catalyst should be at least approximately 50% of the quantity that would be required without the co-catalyst. The concentration of co-catalyst can be selected specifically for each application by routine experimentation. Experience has shown that for the production of transparent foodstuffs packaging such as beverage bottles, a concentration range of 0.1 to 50 ppm (preferably 0.5-15 ppm) of activated charcoal is favorable, whereas to produce carrier foils for the film industry concentrations of 10-500 ppm are advantageous. In order to produce mass-colored, black textile fibers or other molded articles, the concentration can be increased up to 1000 ppm. Still higher concentrations of activated charcoal do not provide further advantages in regard to catalytic activity, but can be taken into consideration with regard to acting as a black colorant.
To produce polyester or copolyester according to the invention through transesterification of at least a dicarboxylic acid dialkyl ester or by esterification of at least a dicarboxylic acid with at least a diol and subsequent single or multi-stage polycondensation, the addition of co-catalyst or activated charcoal and of the usual catalysts required for the polycondensation and, optionally, for the esterification, take place separate from one another or together as a suspension, preferably in the diol that is the base for the polyester. For polyester produced by the esterification process, the activated charcoal suspension can be added before, during, or after the esterification or before or during the first half (relative to dwell time) of the polycondensation. In transesterification process the addition of activated charcoal should take place only after blocking the transesterification catalysts, since the catalytic activity of the activated charcoal for the polycondensation is greatly inhibited by the phosphorus compounds that are usually used for this purpose.
Stabilizers such as phosphoric acid, phosphorous acid, phosphonic acid, carboxyphosphonic acid and their derivatives that are often employed for the production of polyester, in particular for packagings, should not be added at the same time as the addition of the activated charcoal and, relative to the course of the polyester production, be as far apart from it as possible. Thus, for example, the activated charcoal can be added at the beginning of the esterification process to the monomer mixture, and the phosphorus containing stabilizer added at a quantity of 1-50 ppm (preferably 1-10 ppm) phosphorus relative to polyester, at the earliest after completing the supply of the entire monomer mixture, corresponding to a degree of esterification of 60 to 98%. For non-catalyzed esterification, the phosphorus containing stabilizer can be added at the beginning of the esterification, and at the end of the esterification the activated charcoal can be added together with the usual polycondensation catalyst. Other variations are also possible.
Polyesters are hereby to be understood as referring to polymers of terephthalic acid or 2,6-naphthalene dicarboxylic acid and ethylene glycol, 1,3-propandiol, 1,4-butandiol and/or 1,4-cyclohexanedimethanol, and their copolymers with other dicarboxylic acids such as, for example, isophthalic acid, terephthalic acid, 1,4-cyclohexanedicarboxylic acid, 2,6-naphthalindicarboxylic acid, p-hydroxybenzoic acid, 4,4xe2x80x2-bisphenyldicarboxylic acid, adipic acid and/or diols such as diethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and polyglycols with a molecular weight under 1000. The preferred polyester is polyethylene terephthalate, which contains 0.5-5.0 weight % diethylene glycol and 0-5.0 weight % isophthalic acid, 2,6-naphthalene dicarboxylic acid, p-hydroxybenzoic acid and/or 1,4-cyclohexanedimethanol as a comonomer.
Depending on the application of the polyester, it may be advantageous to compensate for the graying of the polyester, that arises from light absorption by the finely distributed activated charcoal, by means of the addition of an optical brightener such as OB1 from Eastman Chemical B.V., The Haag/Netherlands. This is particularly expedient for the production of polyesters for transparent packaging. It may also be advantageous to add a blue toner to the polyester produced according to the invention. Organic blue and blue red dyes in lower ppm quantities, and/or 0-30 ppm cobalt in the form of a salt soluble in polyester are suitable for this purpose. The addition takes place together with the usual catalyst, but can also be done at any desired point in time, in particular at a later point in time.
Also, it is possible to add polyfunctional alcohols such as tri- or tetrahydroxyalkane or polyfunctional carboxylic acids in concentrations of up to 300 ppm relative to the polyester. As a result of this measure, the concentration of the usual catalysts can be reduced even farther. This measure is of particular interest if the polyester is granulated after the melt polycondensation and the granulate is crystallized and post condensed in the solid phase, since an acceleration of the solid phase polycondensation is simultaneously achieved.
As a result of the addition of a co-catalyst according to the invention, the quantity of one or more of the usual catalysts required for the production of a given polyester quality can be reduced by up to half, without impairing the mechanical, chemical, or thermal properties of the polyester. The optical properties of the polyester can be adjusted here in a controlled manner interdependently with the activated charcoal concentration from a minimal change all the way to black coloration, depending on the application.
Polyethylene terephthalate and its copolymers produced according to the invention are distinguished by a carboxyl end group content of less than 40 mmol/kg, preferably 22-38 mmol/kg. Polyethylene terephthalate with 0.5-2 weight % diethylene glycol and 0-5 weight % isophthalic acid and/or 0-5 weight % 1,4-cyclohexanedimethanol, produced with a maximum of 15 ppm of activated charcoal co-catalyst according to the invention and an antimony compound as the usual catalyst, has a turbidity value of less than 10 NTU and is well suited for use as foodstuffs packaging. Generally, the polyester according to the invention produced with the use of 0.1 to 1000 ppm of activated charcoal with the previously indicated properties can be processed into films, bottles and other molded articles.